Adenosine 5'-triphosphate (ATP) is the nucleotide known in biochemistry as the "molecular currency" of intracellular energy transfer; that is, ATP is able to store and transport chemical energy within cells. ATP also plays an important role in the synthesis of nucleic acids. ATP molecules are also used to store energy during the process of photosynthesis, as well as being the store of energy output from cellular respiration. In signal transduction pathways, ATP is used to provide the phosphate for the protein-kinase reactions.

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ATP consists of adenosine and three phosphate groups(triphosphate). The phosphoryl groups, starting with that on AMP, are referred to as the alpha (α), beta (β), and gamma (γ) phosphates. ATP is extremely rich in chemical energy, in particular between the second and third phosphate groups. The net change in energy of the decomposition of ATP into ADP and an inorganic phosphate is -12 kCal / mole in vivo, or inside of a living cell, and -7.3 kCal / mole in vitro, or in laboratory conditions. This massive release in energy makes the decomposition of ATP extremely exergonic, and hence useful as a means for chemically storing energy.

The whole process of oxidising glucose to carbon dioxide is known as cellular respiration as is more than 40% efficient at transfering the chemical energy in glucose to the more useful form of ATP.

ATP is also synthesized through several so-called "replenishment" reactions catalyzed by the enzyme families of NDKs (Nucleoside diphosphate kinases), which use other nucleoside triphosphates as a high-energy phosphate donor, and the ATP:guanido-phosphotransferase family, which uses creatine.

ATP energy is released when hydrolysis of the phosphate-phosphate bonds is carried out. This energy can be used by a variety of enzymes, motor proteins, and transport proteins to carry out the work of the cell. Also, the hydrolysis yields free inorganic Pi and ADP, which can be broken down further to another Pi and AMP. ATP can also be broken down to AMP directly, with the formation of PPi. This last reaction has the advantage of being an effectively irreversible process in aqueoussolution.

The total quantity of ATP in the human body is about 0.1 mole. The energy used by human cells requires the hydrolysis of 200 to 300 moles of ATP daily. This means that each ATP molecule is recycled 2000 to 3000 times during a single day. ATP cannot be stored, hence its consumption must closely follow its synthesis. On a per-hour basis, 1 kilogram of ATP is created, processed and then recycled in the body.

There is talk of using ATP as a power source for nanotechnology and implants. Artificial pacemakers could become independent of batteries. ATP is also present as a neurotransmitter independent from its energy-containing function. Receptors that utilise ATP as their ligand are known as purinoceptors.